Synthetic paper as a substitute for paper, composed mainly of synthetic resins, has excellent properties as compared with paper composed of natural materials, i.e., natural paper, such as water resistance, dimensional stability to moisture absorption, surface stability, glossiness and definition for printing, and mechanical strength. In recent years, practical uses of synthetic paper have been developed by making good use of these advantages.
As the main raw material of synthetic paper, there are used synthetic resins such as polyethylene, polypropylene and polyesters. Among these, polyesters such as polyethylene terephthalate are excellent in the properties such as high thermal resistance and high toughness, so that they can find wide applications.
The following processes have been proposed for production of a polyester film having similar functions to those of paper by using a polyester as the main raw material:
(1) treatment for allowing the inside of a polyester film to contain a large number of fine voids, i.e., preparation of a void-containing polyester film; and PA1 (2) treatment for making rough the surface of a conventional flat polyester film by a technique such as (2-1) sandblasting, (2--2) chemical etching or (2-3) matting (i.e., technique for laminating a matting agent together with a binder).
Among these processes, the process (1) has some advantages in that the film can save its own weight and can be given suitable softness for distinct printing and transfer.
In order to form fine voids in the inside of a polyester film, there has hitherto been proposed a process comprising the steps of: melt-kneading in an extruder, a mixture of a polyester and a polymer which is not compatible with this polyester; extruding the kneaded material into a sheet in which fine particles of the polymer are dispersed in the polyester film base material; and then drawing the sheet to form fine voids around the fine particles (see, e.g., U.S. Pat. No. 5,084,334).
As the polymer which is not compatible with the polyester (hereinafter referred to as incompatible resin), which can be used for the formation of fine voids, there have been proposed numerous resins such as polyolefin resins (see, e.g., Japanese Patent Laid-open Publication No. 134755/1974), polystyrene resins (see, e.g., Japanese Patent Publications Nos. 2016/1974 and 29550/1979) and polyacrylate resins (see, e.g., Japanese Patent Publication No. 28097/1983). Among these resins, polypropylene and polystyrene are particularly preferred, because fine voids can be easily formed by using these resins, and these resins have low density and are available at low cost.
These conventional polyester films, however, have disadvantages in that (1) when provided with an adhesive layer on the surface thereof and used as a label or the like, followed by peeling off, the fine voids present near the film surface act as cracks, so that the surface portion of the film is broken away; (2) they have strong glossiness on the surface thereof, which gives poor appearance for a paper substitute; and (3) the whiteness of the film is decreased by deterioration of the polyester caused by exposure to ultraviolet light or by deterioration arising from by-products of the polyester and the opacifying agent used therein.